APPLICATION NOTE. CryptoAuthentication Personalization Guide. ATSHA204A and ATECC508A. Introduction. Features

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1 APPLICATION NOTE CryptoAuthentication Personalization Guide ATSHA204A and ATECC508A Introduction Before using the Atmel CryptoAuthentication ATSHA204A and ATECC508A devices (crypto devices), there are some initialization processes that need to be performed first. The initialization processes consist of personalizing the device and then locking the device. In the personalization step, the device behavior, the data slot behavior, and the data itself is being configured as desired. After the personalization process is performed, the device needs to be locked to prevent any further modification to data. Details of the initialization processes are described within this document. Features The initialization process consists of four basic steps: Personalize Configuration Zone Personalize device configuration such as I2C_Address, OTPMode, SelectorMode,, UseFlag, LastKeyUse, and Selector Value. Lock Configuration Zone Lock Configuration zone after the Configuration zone has been personalized. Atmel recommends including CRC checksum in the lock process to ensure that the device has been personalized as desired. Configuration zone must be locked to enable the personalization of Data/OTP zones. Prior to locking Configuration zone, Data/OTP zone cannot be read nor written at all. Personalize Data/OTP Zones Personalize Data zone to store data such as keys, calibration data, model number, etc. Personalize OTP zone to store fixed data such as model numbers, calibration information, manufacturing history, or other data that should never change. Lock Data/OTP Zones Lock Data/OTP zones after writing Data and OTP zones. Atmel recommends including CRC checksum in the lock process to ensure the data written are as desired.

2 1 Personalize Configuration Zone There are some bytes that should be configured before using the crypto device. These bytes are used to control the access permission information for each slot of the data memory and to personalize the device behavior itself. The details of these bytes are described below. For more information, please refer to latest ATSHA204A or ATECC508A datasheet. I2C_Address I2C_Address is represented by byte 16 of the Configuration zone. It holds 0xC8 as a default value for ATSHA204A and OxC0 for ATECC508A. This byte is mainly used to identify the device in I 2 C communication. This byte can also be used to control the input level for the crypto devices. OTPMode This OTPMode is represented by byte 17 of the Configuration zone. It holds 0x55 as default value. This byte is used to control the permission level of OTP zones. OTP zone can be used as additional message body for generating MAC, HMAC, or GenDig response, according to the command mode. SelectorMode This bit is used to control the write permission to Selector byte. The SelectorMode is represented by bit 0 of byte 19 within the Configuration Zone. If this bit is cleared, the Selector byte can always be modified with UpdateExtra command. If this bit is set, the Selector byte can only be written if it currently has a value of zero. Slot Configuration Slot Configuration bytes are represented by byte Each slot uses two bytes to determine the slot behavior, byte 20 and byte 21 are the configuration bytes for key slot 1, byte 22 and byte 23 which are the configuration bytes for key slot 2, and so on. Note: The even bytes are LSBytes it holds bits 0:7, while the odd bytes are MSBytes it holds bits 8:15. For example, slot 1 configuration bytes are byte 20 and byte 21, bits 0:7 are hold by byte 20, while bits 8:15 are hold by byte 21. See the below table for details of the 2-byte configurations of each slot. Table byte Slot Configurations Bit Device Description 0 3 Both ReadKey ATSHA204A CheckOnly 4 ATECC508A NoMac Determine which KeyID will be used to generate the encryption key to encrypt the data being read from the corresponding slot. Determine whether the corresponding slot is used for CheckMac command only or can be used for all crypto commands. Determine whether the corresponding slot cannot be used by MAC or HMAC command or can be used for all crypto commands. 5 ATSHA204A ATECC508A SingleUse LimitedUse 6 Both Determine whether the usage of the corresponding slot is limited or not. This limitation only applied to slot 0 thru 7 and 15. Must be zero for keys in Slots 8 thru 14. Determine whether the usage of Slot 15 is limited or not. If this bit is set on Slot 0 thru 14, then any use of the keys will cause counter[0] to increment automatically prior to the operation being performed. Determine that reading from the corresponding slot must be encrypted or not. is represented by bit 6 of the slot configuration. 7 Both Determine that the corresponding slot is secret or not Both WriteKey Both WriteConfig Determine which KeyID will be used to validate and encrypt data written to the corresponding slot. Determine the modification ability of the corresponding slot. There are two ways to modify the data in the slot; by using Write command and by using DeriveKey command. The WriteConfig control the ability of these two commands to modify the data. 2

3 UseFlag / Counter[0:1] For ATSHA204A, UseFlag is represented by even byte from byte 52 to 67. Byte 52 corresponds to Key0, 54 to Key1, and so on. These bytes are used to indicate how many times a key may be used. This limitation is only applied to key slot 0 thru 7. By default, this byte is set to 0xFF. Each time the key is used, a UseFlag bit changed from one to zero and it starts from the most significant bit to the least significant bit. By default, a key may be used eight times before it must be refreshed by using a Write or DeriveKey command; however, these limitations can be reduced by clearing some of the most significant bit, into 0x7F (seven uses), 0x3F (six uses), 0x1F (five uses), and so on. Atmel recommends that the key to be used a single time only, with the other chance of uses providing a safety margin for errors. For ATECC508A, byte 52 to 67 of the Configuration zone are used as Monotonic Counters. Counter[0] can optionally be connected to key 0 thru 14 via the.limiteduse bit. When Counter[0] is attached to a key, the counter will be incremented with each use of the key until the counter has reached its maximum value at which point use of the key will no longer be permitted. The number of legal uses for a key can be controlled by initializing the Counter[0] to a non-zero value at configuration time. Contact Atmel for details. Both Counter[0] and Counter[1] can be incremented via the Counter command. LastKeyUse LastKeyUse is represented by bytes The default value of these bytes is 0xFF. These bytes act similarly like UseFlag, but they are only applied to Key15. Each time Key15 is used, the same mechanism as UseFlag is applied here. Therefore, this key can be limited up to 128 uses. The user can reduce the use limitation by setting these bytes the exact same way as setting the UseFlag byte. The total number of bits set to one indicates the number of usage limitations. After all the LastKeyUse reached 0x00, key15 is permanently disabled. There is no mechanism reset the LastKeyUse bytes. Selector Selector is represented by byte 85. The default value of this byte is 0x00. This byte is used to select which chip will remain in active mode after the execution of pause command. This byte cannot be modified by using normal write command; instead it can only be updated using UpdateExtra command. Key Configuration For ATECC508A, Key Configuration bytes are represented by byte The 16 KeyConfig elements are used in addition to to restrict the actions that can be performed using information stored in a particular slot. The KeyConfig element is interpreted according to the table below when the Data zone is locked. When the Data zone is unlocked, these restrictions do not apply, with the exception that slots configured to contain private keys can be written only with the PrivWrite command., which is represented by bit 2 4 of KeyConfig, must be set on every slot that does not contain an ECC key. 2 Lock Configuration Zone After the crypto device is configured, the next step is to lock Configuration zone. Prior to locking Configuration zone, neither read nor write is permitted to the Data/OTP zones; therefore, the Configuration zone must be locked before personalizing Data and OTP zones. Atmel recommends using CRC checksum in the lock process to ensure the device has been configured as desired. The crypto device uses CRC-16 algorithm to generate a summary digest of the designated zones. For Configuration zone, the CRC is calculated over all bytes (88 bytes in ATSHA204A; 128 bytes in ATECC508A) of the Configuration zone. If the CRC does not match, an error is returned from the device, indicating there is data mismatch. If there is data mismatch, the personalization process needs to be repeated to ensure every personalization is as desired. 3

4 3 Personalize Data/OTP Zones ATSHA204A Data zone consists of 512 bytes split into 16 equal-sized slots. Whereas, the ATECC508A Data zone consists of 1208 bytes split into 16 slots of varying sizes. Each slot access restrictions are individually programmable and can be used to store secret key, calibration data, model number, or other information related to the item to which the crypto device is attached. Additionally, ATECC508A slots can be used to store Private Key, Public Key, Certificate, and/or Signature. While all slots can be used to store Private Key or User Data, only Slots 8 thru 15 are large enough to store an ECC Public Key or ECDSA Certificate/Signature. To be used for Asymmetric Authentication, the ATECC508A support internal generation of Private and Public Key pair via GenKey command. If the Private and Public Key pair is externally generated, the Private Key can also be stored using PrivWrite Command. Other data aside from the Private Key can be stored using Write command. OTP zone consists of 64 bytes of one-time programmable (OTP) bits. The OTP zones can be used to store fixed data such as, model numbers, calibration information, manufacturing history, or other data that should never change. These bytes can freely be written after the Configuration zone has been locked, but prior to Data/OTP zones locked. 4 Lock Data/OTP Zones Upon completion of any writes, the Data and OTP sections should be locked. It is important that the Data and OTP sections be locked prior to release of the system containing the device into the field. Failure to lock these zones may permit modification of any secret keys and may lead to other security problems. Atmel recommends using CRC checksum in the lock process to ensure the data written are as desired. The crypto device uses CRC-16 algorithm to generate a summary digest of the designated zones. For the Data and OTP zones locked, the contents are concatenated in the order to create the input to the CRC algorithm. If the CRC does not match, an error is returned from the device, indicated that there is data mismatch; therefore the personalization process needs to be repeated. 5 Personalization Example This section gives an example of the desired device personalization and the corresponding bits setting for the device. 5.1 Accessory Authentication Use Case Two crypto devices are used in an accessory authentication, such as battery authentication. In this case, the crypto device is embedded in a mobile device as Host and in the battery as Client. The symmetric diversified key scheme is also utilized in this example case. The communication protocol used between MCU and the crypto device is I 2 C protocol. 4

5 5.1.1 Host Crypto Device Personalization The personalization of the Host for this example case is described as follows. I2C_Address Set this byte as 0xAA. Another value can be used as long as it is different from the Client crypto device I2C_Address. for the Master Key Slot. (i.e. slot 0) This key is used to generate diversified key, which is used in the authentication process; therefore, the key must be configured to be No Read or Write permitted on this slot. The for this slot is 0x See the below table for more details. Table 5-1. for the Master Key Slot Must be set to zero to enable all crypto functions on this slot. Must be set to zero to disable the limited usage. WriteKey Can be set to any value, i.e. 0x0. WriteConfig Must be set to disable Write and DeriveKey command, i.e. 0x8. for the Diversified Key Slot (i.e. slot 1) Used to check the authenticity of the accessory. The key must be configured to be No Read, while the content can only be modified by deriving from Master Key. The for this slot is 0x Table 5-2. for the Diversified Key Slot Must be set to one, since it is only used for performing authentication with the accessory. Must be set to zero to disable the limited usage. WriteKey Must be set to Master Key, in this case, 0x0. WriteConfig Can be set to 0x3 or 0xB depends on the authorization is required or not, in this example, it is set to 0x3. Other settings can be left as default. 5

6 5.1.2 Client Crypto Device Personalization The personalization of the client for this example case is shown below. I2C_Address Set this byte as 0xBB. Other value can be used as long as it is different from the Host crypto device I2C_address. for the Diversified Key Slot (i.e. slot 0) Used to generate response for the authentication process. The key must be configured to be No Read or Write permitted on this slot. The for this slot is 0x Table 5-3. for the Diversified Key Slot Must be set to zero to enable all crypto functions on this slot. Must be set to zero to disable the limited usage. WriteKey Can be set to any value, i.e. 0x0. WriteConfig Must be set to disable Write and DeriveKey command, i.e. 0x8. Other settings can be left as default. 6

7 5.2 Consumable Authentication Use Case In this case, the crypto device is used to authenticate a consumable, track the consumable uses, and also limit the consumable uses. The crypto device is embedded in the Host and also in the consumable. In this example, the communication protocol being used is SWI interface Host Crypto Device Personalization The personalization of the Host for this example case is described below. SelectorMode Can be set to any value other than zero to prevent further modification of Selector byte. Slotconfig for the Authentication Key Slot (i.e. slot 0) Used to check the authenticity of the consumable. The key must be configured to be No Read or Write permitted on this slot. The for this slot is 0x See the below table for details. Table 5-4. Slotconfig for the Authentication Key Slot Must be set to one to enable only CheckMac command. Must be set to zero to disable the limited usage. WriteKey Can be set to any value, i.e. 0x0. WriteConfig Must be set to disable Write and DeriveKey command, i.e. 0x8. Selector Set this byte as 0xAA. Other value can be used as long as it is different from the Client crypto device Selector byte. Value 0x00 is prohibited to prevent further modification on this byte. Other settings can be left as default. 7

8 5.2.2 Client Crypto Device Personalization The personalization of the Host for this example case is described below. SelectorMode Can be set to any value other than zero to prevent further modification of Selector byte. for the Authentication Key Slot Must use Slot15. Used to generate the response for the authentication process. The key must be configured to be No Read or Write permitted on this slot and also configured to be used for a limited use only, i.e. 64 uses. The for this slot is 0xA1 80. See the following table for details. Table 5-5. for the Authentication Key Slot Must be set to zero to enable all crypto functions on this slot. Must be set to one to enable the limited usage. WriteKey Can be set to any value, i.e. 0x0. WriteConfig Must be set to disable Write and DeriveKey command, i.e. 0x8. LastKeyUse Set these bytes to 0x FF FF FF FF FF FF FF FF. These bytes limit the usage of KeyID 15 to 64 uses. Selector Set this byte as 0xBB. Other value can be used as long as it is different from the Host crypto device Selector byte. Value 0x00 is prohibited to prevent further modification on this byte. Other settings can be left as default. 6 Revision History Doc Rev. Date 8845C 07/2015 Added the ATSHA204A and ATECC508A devices. 8845B 05/2013 Simplified the document and added another example. 8845A 11/2011 Initial document release. 8

9 Atmel Corporation 1600 Technology Drive, San Jose, CA USA T: (+1)(408) F: (+1)(408) Atmel Corporation. / Rev.:. Atmel, Atmel logo and combinations thereof, Enabling Unlimited Possibilities, CryptoAuthentication, and others are registered trademarks or trademarks of Atmel Corporation in U.S. and other countries. Other terms and product names may be trademarks of others. DISCLAIMER: The information in this document is provided in connection with Atmel products. No license, express or implied, by estoppel or otherwise, to any intellectual property right is granted by this document or in connection with the sale of Atmel products. EXCEPT AS SET FORTH IN THE ATMEL TERMS AND CONDITIONS OF SALES LOCATED ON THE ATMEL WEBSITE, ATMEL ASSUMES NO LIABILITY WHATSOEVER AND DISCLAIMS ANY EXPRESS, IMPLIED OR STATUTORY WARRANTY RELATING TO ITS PRODU CTS INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTY OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT. IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE, SPECIAL OR INCIDENTAL DAMAGES (INCLUDING, WITHOUT LIMITATION, DAMAG ES FOR LOSS AND PROFITS, BUSINESS INTERRUPTION, OR LOSS OF INFORMATION) ARISING OUT OF THE USE OR INABILITY TO USE THIS DOCUMENT, EVEN IF ATMEL HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. Atmel makes no representations or warranties with respect to the accuracy or completeness of the contents of this document and reserves the right to make changes to specifications and products descriptions at any time without notice. Atmel does not make any com mitment to update the information contained herein. Unless specifically provided otherwise, Atmel products are not suitable for, and shall not be used in, automotive applications. Atmel products are not intended, authoriz ed, or warranted for use as components in applications intended to support or sustain life. SAFETY-CRITICAL, MILITARY, AND AUTOMOTIVE APPLICATIONS DISCLAIMER: Atmel products are not designed for and will not be used in connection with any applications whe re the failure of such products would reasonably be expected to result in significant personal injury or death ( Safety-Critical Applications ) without an Atmel officer's specific written consent. Safety-Critical Applications include, without limitation, life support devices and systems, equipment or systems for the operation o f nuclear facilities and weapons systems. Atmel products are not designed nor intended for use in military or aerospace ATSHA204A applications and or environments ATECC508A unless specifically Personalization designated by Atmel Guide as military-grade. [APPLICATION Atmel products NOTE] are not designed nor intended for use in automotive applications unless specifically designated by as automotive-grade. 9